a. Field of the Invention
The present invention is directed towards a method for continuously calibrating an optical vibration sensor and, more particularly, to a calibration technique which continuously compensates for changes in the resonant frequency of the optical vibration sensor due to temperature and mechanical changes of the optical vibration sensor.
b. Description of the Related Art
Generator end-windings producing 60 hertz (Hz) electrical current, continuously vibrate at 120 Hz. Generator end-winding vibration has been effectively monitored using fiber optic technology with end-turn vibration monitoring systems. The end-turn vibration monitoring systems utilize optical vibration sensors and a monitoring system to convert light pulses into a measured vibration amplitude. Optical vibration sensors are required because of the high electric potential, e.g., approximately 20 Kilovolts, associated with the generators. The capability to monitor stator end-winding vibration levels during operation provides for the identification of changes occurring within the generator, thus allowing the opportunity for early correction of damaging conditions before extensive winding repairs or replacement are necessitated. Further, the information obtained by vibration monitoring can be compiled over a period of time and compared with other data, thus permitting in-depth assessment of the generator's current condition, and allowing enhanced maintenance planning.
A problem exists in that the optical vibration sensors of the prior art are not easily calibrated. That is, the vibration sensors suffer from a significant temperature drift causing a change in the resonant frequency of the vibration sensors which causes inaccurate measurement of the vibration amplitude. These optical vibration sensors are available, for example, from the Westinghouse Corporation located in Orlando, Fla. as Model 9299A43. These type of sensors include an internal reed which vibrates a grid assembly. These grid assemblies are typically etched using the same photographic techniques used to produce integrated circuit chips.
Not only are the vibration sensors affected by changes in temperature, but the sensors are also affected by metal fatigue or loss of mass. For example, the internal reed of the vibration sensor starts to fatigue after several million cycles. As a result, the resonant frequency changes as the elastic parameter of the internal reed changes. Also, a loss of mass could occur if a small piece of metal, solder, epoxy or the like used to construct the vibration sensor were to fall off or become separated from the internal reed or grid assembly of the vibration sensor.
The prior art technique for compensating for changes in temperature is to calibrate the vibration sensor at the average temperature at which the vibration sensor will be used. This approach is somewhat effective but requires recalibration when changing environments. Further, the actual vibration magnitude obtained using the known calibration technique suffers from a loss of accuracy. Further, known calibration techniques cannot compensate for metal fatigue, temperature change from the average temperature, loss of mass or the like.